1. Field of the Invention
The present invention relates in general to an electrically operated parking brake apparatus provided on an automotive vehicle, and more particularly to techniques for improving the usage or operational flexibility of the parking brake apparatus.
2. Discussion of the Related Art
Generally, a parking brake as one of braking actions is mechanically applied to an automotive vehicle, by an operation of a parking brake lever or pedal by the operator of the vehicle. On the other hand, there is known an electrically operated parking brake apparatus for electrically applying a parking brake to the vehicle, as disclosed in JP-A-4-108058. The electrically operated parking brake apparatus requires a reduced operating force applied by the vehicle operator to the parking brake lever or pedal, in order to apply the parking brake to the vehicle.
In a mechanically operated parking brake apparatus adapted to mechanically apply a parking brake to a vehicle, the braking force (hereinafter referred to as "parking brake force") produced by a parking brake device can be increased or reduced, or the parking brake can be released, even when a drive power source switch (e.g., an ignition switch or main power source switch) is placed in a non-operated position. The drive power source switch is operated to an operated position to turn on the drive power source (e.g., an engine or electric motor) of the vehicle. However, the above-identified publication does not teach that the electrically operated parking brake apparatus is capable of increasing or reducing the actual value of the operating force of the parking brake device (parking brake force produced by the parking brake device), or releasing the parking brake, while the drive power source switch is placed in its non-operated position. While the electrically operated parking brake apparatus disclosed in the publication is advantageous, for its reduced operating force, over the conventional mechanically operated parking brake apparatus, the electrically operated parking brake apparatus suffers from a comparatively low degree of usage or operational flexibility.
In the electrically operated parking brake apparatus disclosed in the above-identified publication, all of brakes provided for front left and right wheels and rear left and right wheels of the vehicle are used as the parking brake devices. Accordingly, the total parking brake force produced by the system is relatively large even with a relatively small parking brake force produced by each parking brake device. However, the electrically operated parking brake apparatus having such an advantage is less flexible in use than the conventional mechanically operated parking brake apparatus. That is, the mechanically operated parking brake apparatus generally uses only two parking brake devices for the rear left and right wheels of the vehicle, so that the vehicle operator may positively utilize these rear parking brake devices, for permitting a quick turn of the vehicle, namely, to cause a so-called "spin turn" of the vehicle by activating the parking brake devices of the rear wheels during running of the vehicle. This way of operation of the parking brake devices cannot be accomplished in the conventional electrically operated parking brake apparatus.